I just downloaded it and played it a bit last night, actually. GLQuake is a 3d accellerated version, so it looks better than the original version of quake. Runs plenty fast on my 1ghz iBook with 256megs of RAM, and I think I remember playing it on my PIII 450, so you shouldnt have trouble running it on whatever you throw at it.
http://macglquake.sourceforge.net/
Download the glquake stuff, download the shareware quake files, copy the glquake executable into the shareware quake directory. It will give you episode I.
For the full version, you will have to probably install on a PC and copy the files over, or figure out where all the files go yourself and copy them off the CD.